Layered current sensor

ABSTRACT

An electrical current sensor and utility electricity meter, the current sensor comprising a π resistor shunt configuration, wherein the resistors comprise layered conductors at substantially equal temperatures to provide a zero temperature coefficient sensor. A fiscal electricity meter is described together with a four-layered current sensor fabricated using PCB techniques.

FIELD OF THE INVENTION

[0001] This invention relates to the field of electrical currentsensing, particularly but not exclusively for use as a current shuntsensor used in a fiscal utility electricity meter.

BACKGROUND

[0002] Electrical current sensing is used in many applications. Thereare two common methods used to sense the current. The first methoddetects and measures the magnetic field produced by the current flowingin a conductor (inductive systems). For example U.S. Pat. No. 4,794,326describes an arrangement comprising a planar conductor coupledinductively by a transformer arrangement to the sense circuit. A secondmethod of current sensing uses a ‘shunt’ resistance in the current pathto generate a voltage across the shunt resistance in proportion to thecurrent flowing. In U.S. Pat. No. 5,701,253 a utility electricity meteremploying a current shunt is described.

[0003] In U.S. Pat. No. 4,492,919, a planar current shunt is described.The invention incorporates an electrical transformer to provideisolation between the conductor carrying the current and the sensingcircuit.

SUMMARY OF THE INVENTION

[0004] According to the present invention there is provided a layeredcurrent divider comprising a conductor carrying the electrical currentand two resistors, all the resistors mounted to occupy substantially thesame physical area in a layered structure, wherein the temperaturedifference between the layers is substantially zero.

[0005] The layered structure provides electrical isolation between themain current conductor and the two resistors, whilst providing a thermalconductive path between the layers. This ensures that for any smallportion of the main current carrying conductor area, there is a similararea portion on the two other resistors and all three areas aresubstantially at the same temperature. This ensures that the currentdivider has substantially zero temperature co-efficient independent ofthe temperature co-efficient of the metal resistivity comprising thecurrent carrying conductor and two other resistors.

[0006] For optimum thermal performance, the main current carryingconductor may be divided into two equal layers. The heat generating mainconductor comprises two outer layers and the two other, non-heatgenerating, resistors comprise the inner layers. The layered structureis isothermal across the thickness.

[0007] According to a further aspect of the invention there is provideda current sensor for measuring electrical current flow, comprising alayered current divider and a low input impedance differential currentamplifier. The amplifier produces a large signal output directlyproportional to the current flowing through the current divider.

[0008] One benefit of the current divider when fabricated from PCB isthe ability to integrate the current divider with existing electronicsmanufactured using PCB interconnect technology. The current divider canbe integrated directly onto the same PCB used to interconnect theelectrical components.

[0009] According to a further aspect of the invention there is providedan electricity meter for measuring the electrical power consumed by aload, comprising a layered current sensor, a voltage sensing means and apower computation means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

[0011]FIG. 1 illustrates the current divider element,

[0012]FIG. 2 illustrates the electrical circuit of the π configurationcurrent divider,

[0013]FIG. 3 shows the conductor pattern for the layers within thecurrent divider,

[0014]FIG. 4 shows in detail the conductor pattern for the firstresistor,

[0015]FIG. 5 shows in detail the conductor pattern for the secondresistor,

[0016]FIG. 6 is the circuit schematic for the electricity meter.

Detailed Description

[0017] The current divider element is shown in FIG. 1 and comprises a 4layer printed circuit board (PCB), 1. The measured current is connectedto the device by means of copper bars 2 and 3. The current divideroutput is copper pads 4 and 5. The current divider may be connected to alow input impedance differential current amplifier circuit that providesadequate gain to produce a voltage output signal.

[0018] For optimum temperature performance, the current density in thecurrent carrying resistor should be uniform. This is achieved by copperbars 2 and 3.

[0019]FIG. 2 is the electrical circuit diagram of the current divider.The sensed current flows though resistor 6. Two resistors, 7 and 8, areconnected to resistor 6 and have outputs 4 and 5.

[0020]FIG. 3 illustrates the four conductor patterns comprising the PCB,1. The total PCB thickness is 1mm and the inner layers are ˜80 μm apart.The top layer, 9, and the bottom layer, 10, comprise two copper tracks,11 and 12, providing a low resistance main conductor path that isresistor 6. In one embodiment the copper track, 11 and 12 are 2.5 cmsquare and fabricated from 5 ozin (175 μm) copper foil. The resistanceof the main conductor, resistor 6, is approximately 50 μohms.

[0021] The two resistors, 7 and 8, comprise track patterns on the innerPCB layers 13 and 14. The copper thickness of layers 13 and 14 is 5 μmand each resistor comprises ˜150 cm of track 75 μm wide distributed overan area identical to that occupied by the main conductor tracks, 11 or12. The resistance of resistors 7, 8 is approximately 70 ohms.

[0022] The electrical output connections to the resistors 7 and 8 are bycopper area (pads) 4 and 5. The resistors are connected to the mainconductor at points 15 and 16 on layer 9. Similar connections exist onlayer 10. The conductor area occupied by the track 11 that comprisesresistor 6 is defined by the physical positions of connection points 15and 16 and the track width.

[0023]FIG. 4 and FIG. 5 show in detail the track patterns of theresistors, 7 and 8. The track, 17 and 18 comprises a ‘zig-zag’ patternfolded into a PCB area identical to that occupied by resistor 6.Electrical interconnects are provided by vias at the appropriatepositions to provide the circuit shown in FIG. 2. The electricalconductor paths are arranged with substantially zero inductive couplingbetween the resistors, 7 and 8, and the current flowing in the resistor6. Further, the track patterns 17 and 18 have substantially zerocoupling with an a.c. magnetic field vector in any direction that hasuniform magnitude across the area of the current sensor.

[0024] The current divider is interfaced to low input impedancedifferential current amplifier circuit that provides a voltage outputsignal. This amplifier provides an output signal proportional to thedifference in current flowing though resistors 7 and 8. Such amplifiercircuits are well known in the art.

[0025] The inventors have found that this embodiment has substantiallyzero temperature co-efficient over the operating temperature range of anelectricity meter. The temperature co-efficient measured isapproximately {fraction (1/200)}^(th) that of the copper conductormaterial.

[0026] The outer conductors in this embodiment can carry several hundredamps and therefore dissipate heat. The inner resistor layers do notcarry substantial current and therefore do not generate heat. The fourlayer design has substantially zero temperature gradients across thethickness of the current divider. When combined with a low impedancedifferential current amplifier, the resulting current sensor hassubstantially zero temperature co-efficient.

[0027]FIG. 6 illustrates the circuit diagram of an electricity meter.The circuit is well known in the art and the function is summarisedbelow. Further details of this circuit may be found in themanufacturers' application note. The meter measures the power consumedfrom a supply, 19, by a load, 20. The current divider comprisesresistors 21, 22 and 23. A standard integrated circuit (I.C.), 24, oftype SA9602, manufactured by SAMES of South Africa provides theprocessing functionality to compute power drawn by the load. The ICprovides a low impedance differential current amplifier circuit tomonitor the current flowing from the supply to the load circuits. Thecurrent sensor division ratio is defined by the ratio of resistors 21and 22, 23.

[0028] The division ratio is a function of the conductor patterngeometry and with the embodiment described the ratio is ˜250,000 to1,000,000 to 1. The IC further measures the voltage across the sourceand computes the power consumed from instantaneous current and voltagemeasurements. The power measured is output by means of pulses that areinterfaced using an opto-isolator, 21, to provide output signalisolation from the supply voltages.

FURTHER EMBODIMENTS

[0029] The layered current divider element described above ismanufactured using standard PCB processes. Therefore the resistors aremade from copper and epoxy fibre-glass provides electrical isolationbetween the layers. The current divider geometry can be changed in sizeas required to change the current division ratio. The current dividerresistors can be computed from the track widths, thickness and lengths.The conductors may be manufactured from any conductor material providedthe temperature co-efficient is substantially identical on each layer.The current divider structure may be manufactured using alternativemethods other than chemical etching. The conductor paths may bemanufactured by stamping from copper. The current divider layeredstructure can be achieved by suitable mechanical bonding of separatelymanufactured layers.

[0030] The current divider need not be limited to a four layerembodiment. In one embodiment, the device comprises two layers, withresistor 6 on one layer and resistors 7 and 8 comprising a second layer.

[0031] In a further three layer embodiment, the track comprisingresistors 7 and 8 are shared between two copper layers on one PCB. Thetwo layer PCB is bonded to a copper conductor comprising resistor 6.Suitable electrical connections are made to achieve the circuitconfiguration shown in FIG. 2. Similarly mote than four layer structuresare conceivable, wherein each resistor comprising the current divideroccupies two or more layers

[0032] The current divider may be manufactured directly onto integratedcircuits using multiple layer metal interconnects. IC manufacturingprocesses using mote than two layers of metal or interconnects arecommon.

[0033] In a final embodiment, the effect of non-uniform current density(J) in the main current carrying resistor, 6, is corrected. The otherresistors comprising the current divider have a uniform track densityover their layer area. Adjustments in the track to track spacing affectthe track density. The current density in resistors 7 and 8 can beadjusted by altering the track density to match the current density inresistor 6 and provide a zero temperature coefficient current divider.Similarly the conductor thickness may be adjusted to provide the sameeffect.

1. A layered electrical current divider comprising a shunt resistor andtwo resistors in a π configuration, wherein the material comprising theresistors is thin and flat, has substantially identical temperaturecoefficients and the resistors are arranged such that the resistancesare at substantially equal temperatures.
 2. A layered electrical currentdivider according to claim 1, wherein the areas of each layer, overwhich the conductor paths comprising the resistors ate distributed, aresubstantially equal.
 3. A layered electrical current divider accordingto claim 2, comprising at least three layers, wherein the sensed currentflows in a resistor comprising one layer and the other resistors aredistributed on at least two other layers.
 4. A layered electricalcurrent divider according to claim 2, comprising four layers, whereinthe sensed current flows in a resistor comprising two layers and theother resistors are distributed on two layers.
 5. A layered electricalcurrent divider according to claim 2, comprising four or more layers,wherein the sensed current flows in a resistor comprising the twooutside layers and the other resistors are distributed on two or moreinner layers.
 6. A layered electrical current divider according to anyof claims 1 to 5, wherein the conductor paths comprising the non-sensedcurrent-carrying resistors have substantially zero inductive coupling toa uniform magnetic field.
 7. A layered electrical current divideraccording to any of claims 1 to 5, wherein the conductor pathscomprising the non-sensed current-carrying resistors have substantiallyzero inductive coupling to the current carrying resistor.
 8. A layeredelectrical current divider according to any of claims 1 to 7, whereinthe conductor paths are manufactured using PCB techniques.
 9. Anelectrical current sensor comprising a layered electrical currentdivider according to claim 8, wherein the current divider is integratedonto a PCB providing interconnection between electrical components. 10.An electrical current sensor comprising a layered electrical currentdivider according to any one of claims 1-8
 11. A method of sensingcurrent using a low impedance differential current amplifier and acurrent divider, comprising at least one layer in which the measuredcurrent flows and at least one layer comprising two resistors, whereinlayer areas over which the resistors are distributed are substantiallyequal temperature and equal in area.
 12. An apparatus for measuringelectrical power comprising: voltage sensing means, power consumptioncomputation means, a layered electrical current divider, comprising ashunt resistor and two resistors in a π configuration, wherein thematerial comprising the resistors has substantially identicaltemperature coefficients and the resistors arranged such that the areasoccupied by the resistors are substantially equal in temperature and areequal in area, differential current amplifier means.
 13. An apparatusfor measuring electrical power according to claim 12, wherein thecurrent divider comprises three layers, and the sensed current flows ina resistor comprising one layer and the other resistors are distributedon at least two other layers.
 14. An apparatus for measuring electricalpower according to any of claims 12 to 13, wherein the current divideris integrated onto the PCB containing the electrical interconnectbetween the apparatus electrical components.
 15. An apparatus formeasuring electrical power according to claim 12, wherein the currentsensor comprises four or more layers, and the sensed current flows in aresistor comprising at least two layers and the other resistors aredistributed on two or mote layers.